Field of the Invention
The invention concerns a method for creating magnetic resonance angiography images of an examination volume of a patient using time-of-flight angiography in a magnetic resonance apparatus wherein, by continuous recording, a number of two-dimensional slice images covering the examination volume along an axial direction are acquired slice-by-slice, particularly in an overlapping manner. The invention also concerns a magnetic resonance apparatus and a non-transitory, computer-readable data storage medium encoded with programming instructions computer program to implement such a method.
Description of the Prior Art
Magnetic resonance imaging is a known modality for creating magnetic resonance angiography images. Two fundamentally different approaches exist for that purpose. In a first approach, prior to creating the magnetic resonance images, a contrast agent is injected into a subject with which the vessels in which the contrast agent flow being clearly highlighted thereby. Because the administration of contrast agent in modern medical imaging is preferred to be avoided as much as possible, procedures are also proposed in which it is possible to dispense with injecting a contrast agent (frequently referred to as “non-contrast MR angiography”). The best-known example of this is known as time-of-flight angiography with a magnetic resonance apparatus. This exploits the fact that after a saturation of spins is achieved in one slice, “fresh” blood that flows into this slice, and which has not experienced the high frequency pulses resulting in the saturation, has a significantly higher signal intensity. Ideally only the signal of blood that is flowing into the slice is measured.
It is particularly advantageous here for the measurement to take place in the arterial phase, since arterial blood is pumped through the arteries in a pulsatile manner. The arterial phase is understood to mean the phase of the cardiac cycle in which the heart pumps the blood into the arteries. By contrast, the venous phase is the phase of the cardiac cycle in which the blood is fed back via the veins to the heart. In the arterial phase, blood with non-saturated spins therefore quickly penetrates the vessels in the examination volume, so that these can be seen particularly clearly.
It was therefore proposed to perform the recording (acquisition) of the magnetic resonance angiography images in a triggered manner, in particular on the basis of an EKG (electrocardiogram). In this way the arterial phase can be identified chronologically so that the measurement then ideally takes place when blood with non-saturated spins flows through the blood vessels in the examination volume in large quantities. When EKG signals are used to trigger the time-of-flight magnetic resonance angiography, a series of disadvantages nevertheless exists. Couplings of the radio-frequency signals generated by the magnetic resonance scanner into the EKG device are possible, and may result in incorrect actuation signals on the part of the EKG device. A further problem occurs when the patient has a variable heartbeat (arrhythmia), since problems can then occur during data acquisition, particularly when the data acquisition requires determining the phase. A delay time must be planned as precisely as possible, since the further the blood vessels to be examined are from the heart, the longer the blood needs in order actually to enter into the examination volume. These delay times are often extremely difficult to estimate. Finally, EKG actuation signals and the corresponding arterial or venous phase at distal segments of a hemovascular tree only have minor correlations, so that the EKG actuation signals, especially with existing standard deviations in the physiology of the patient, to which the examination applies if necessary, do not conform optimally to the respective phases.
US 2013/0119983 A1 proposes a method for generating angiography images, which can determine angiography images, in particular angiography images of the arterial phase, without using an EKG device or a trigger signal. It is proposed to divide the magnetic resonance data in k-space into different groups in respect of the cardiac cycle, by a Fourier transformation of the detected magnetic resonance data being performed in the temporal respect in order to generate a frequency spectrum of the magnetic resonance data. A corresponding analysis of the frequency spectrum, in particular of the temporal course of the averaged energy from detected radii during radial scanning of k-space, allows the arterial phase to be differentiated from the venous phase and magnetic resonance angiography images in the position space can be generated from the magnetic resonance data of the arterial phase. This analysis is relatively complicated and not always completely reliable.